1. Field of the Invention
The present invention relates to borehole telemetry systems and more specifically to method and apparatus for transmitting data up a well such as an oil or gas well.
2. Brief Description of the Prior Art
Various techniques have been used for sensing parameters, such as pressure, temperature, inclination, etc., downhole in oil and gas wells and for obtaining data about the parameters uphole.
Parameters have been sensed and recorded on strip chart recorders downhole. A problem with this technique is that the recording device must be brought back uphole to be read and, therefore, the parameter being sensed cannot be monitored uphole on a real-time basis.
Techniques have been developed for measuring parameters and transmitting data about the parameters uphole on a real-time basis. One technique is referred to as the soda straw technique in which a small tube extends down in the well casing from the top of the well to the bottom zone where pressure is sensed. An instrument is used to sense the pressure at the top of the tube which gives a measure of bottom hole pressure. Disadvantages of this technique are the high cost and time required to run in and remove the tube from the well, the danger that the tube will create problems with fracturing fluid, increased pressure required to force fluids down the casing due to the introduction of the tube, and high fluid pressure at the top of the well, creating the likelihood of a blowout. These problems are likely to occur when fracturing fluids are pumped between the casing and tubing.
Another technique is one where mud pulses are used to create data pulses in the mud being pumped downhole and the data pulses are sensed uphole. The bits of information per unit time is quite low with this technique and the devices are generally costly and mechanically complex.
Wire line techniques are used where electrical signals are transmitted uphole on a wire or electrical conductor. However, this requires a special wire extending from the surface to the bottom of the hole. Examples of such methods are described in Leonardon, U.S. Pat. No. 2,242,612, Cowles, U.S. Pat. No. 4,035,763, Wilson et al., U.S. Pat. No. 3,434,046, Planche et al., U.S. Pat. No. 4,286,217, and Jakosky, U.S. Patent Reissue No. RE. 21,102.
Other techniques are known for transmitting electrical signals to the top of the well which do not require a wire line. Examples of these techniques will now be discussed.
In an article in the IEEE, "Transactions on Geoscience and Remote Sensing", Vol. GE-20, No. 2, April 1982, J. Bhagwan and F. N. Trofimenkoff, report an electric drill stem telemetry method. Bhagwan et al. describe the use of a main drill stem and a downhole electrode electrically isolated from the main drill stem for transmitting data from downhole to the surface. The main drill stem and the downhole electrode comprise a portion of an electrical circuit, the balance of which includes a distant electrode placed in the earth, a conductor connecting the main drill stem to the distant electrode, and a current path through the earth between the distant electrode and the main drill stem and isolated electrode.
Two methods of telemetry are discussed. The first is a resistance change method wherein the main drill stem and the isolated downhole electrode are alternately connected and disconnected while the resultant resistance change due to the connection or disconnection is monitored at the earth's surface. In the second method, a signal from a downhole signal source is applied between the downhole electrode and the main drill stem, and received by a receiving electrode, placed between the main drill stem and the earth at the surface.
The Bhagwan article is largely theoretical in nature and is deficient in technical details. Several difficulties arise with the first or resistance method. For example, a separate drill stem is required in the cased well. Also, a bottomhole electrode, electrically separated from the drill stem, must somehow be positioned downhole but Bhagwan does not say how this would be done. Also if resistance is measured at the top of the hole using an ohm meter, ohm meters typically employ D.C. signals which would cause polarization along the drill stem. Also Bhagwan teaches that this approach would be difficult to do under field conditions that are normally encountered in drilling or testing situations.
With Bhagwan's downhole signal method, provision must be made downhole for a source of power adequate to transmit signals uphole for substantial periods of time and is not desirable for downhole equipment which must remain downhole for substantial periods of time.
Silverman, U.S. Pat. No. 2,400,170, shows a drill pipe containing an insulated section separating the main drill pipe from the drill collar and drill bit. Electrical waves are transmitted through make and break contacts from the insulated section through the surrounding earth to sensor electrodes located uphole on the surface.
Other methods of telemetry are known for producing an electrical signal downhole and radiating the signal through the earth to sensors located uphole at the surface. Such are the patents to Clark et al., U.S. Pat. No. 1,991,658 and to Subkow et al., U.S. Pat. No. 2,225,668.
Johnston, U.S. Pat. No. 3,437,992, discloses a self-contained downhole parameter signaling system of the type which generates signals downhole for transmission and detection uphole. Johnston discloses a complicated power generating system which uses the movement of a sucker rod connected to a pump and a transformer for generating electrical power downhole for the instrument package. Using the generated power, a circuit applies electrical impulses, representative of downhole parameters such as pressure or temperature, to the primary of a transformer, the secondary of which is connected between the tubing and casing. The connection to the casing is made through a sleeve, which is insulated from the tubing, and outwardly movable leaf spring contacts which engage and electrically connect to the inside of the casing. The impulses which are transferred from the primary to the secondary of the downhole transformer create electrical signals which travel up the tubing and casing to an uphole transformer. The uphole transformer amplifies the signals for conversion to usable form at the top of the well. As a result, Johnston is quite complicated.
Drilling strings are also known with nonconductive sections for electrically separating the drill string into upper and lower electrically conductive drill strings to allow the radiation of signals to the top of the well such as disclosed in Oil & Gas Journal, Feb. 1, 1983, pp. 84-90.
A large source of power is required to maintain both the last two mentioned downhole equipment.